Leak detector systems having mass spectrometers tuned to detect the presence of a test gas which is indicative of a leak are well known. U.S. Pat. No. 3,690,151 discloses such a system in which helium test gas passes through a gas sample inlet into a mass spectrometer via a high vacuum pump associated with the mass spectrometer. A detectable portion of the helium test gas passes through the high-vacuum pump in the direction opposite to the flow of gas being pumped from the mass spectrometer. This reverse flow is attributed to the high velocity, small size and chemical inactivity of the helium atoms, which enables large numbers of them to escape the pumping action of the high-vacuum pump. The above-identified patent also discloses as prior art therein a leak detection system in which the helium test gas passes to the mass spectrometer without going upstream through a high vacuum pump.
The output reading of the mass spectrometer provides an indication of the existence of a leak. In order to prepare the above-described system for operation, it has heretofore been the practice to provide communication between the gas sample inlet and a so-called "standard leak" which comprises a container of substantially pure helium gas, a porous-to-helium membrane, and a valve, all of which is capable of providing a controlled flow rate of helium. The helium introduced into the mass spectrometer by the standard leak is used to tune and calibrate the mass spectrometer. The problem is that standard leaks are expensive, and particularly so if they were made in a size sufficient to provide enough helium to tune a system in which the helium must pass upstream through a high vacuum pump so only a portion of it reaches the mass spectrometer.